[caw]

There’s actually a “bookmark this tweet” option under the more options menu for the tweet.

[tom_]

Bizarrely, this appears to be missing from the main site...

[thrower123]

Ah, I basically only use the Android app, and this is hidden under the button that one expects would launch the confusing and awful Android sharing dialog.

[mayankkaizen]

This is kinda useless. You can bookmark a tweet. But when you undo this bookmarking, all other bookmarked tweets also go.

[tenryuu]

I don't have this option on tweetdeck, only a add to collections, gives me a popup with no content. Nice.

[caw]

It’s on the official site, I’m not sure of 3rd party app support. To view your bookmarks, https://mobile.twitter.com/i/bookmarks

[jdck1326]

I only see it on the mobile site.

It's found to the right of the heart button.

[tenryuu]

tweetdeck is technically first party, twitter acquired the team many many years ago